This application seeks continued support for the Predoctoral and Postdoctoral Training Program in Immunobiology at the Yale University School of Medicine. The program involves 28 trainers from a total of 8 different departments. Sixteen of the trainers hold primary or secondary appointments in the Section of Immunobiology, which is the principal administrative and training entity. Three trainers hold appointments in the Department of Pathology, while the remainder are members of the Graduate Program in Microbiology, which is the second important training entity. The number of graduate students and postdoctoral associates currently in these training programs is 42 and 85, respectively (Immunobiology) and 91 and 148, respectively (Microbiology). The program offers training in virtually all aspects of molecular, cellular and organismal immunology, as well as host-pathogen interactions and the strategies adopted by microorganisms to evade immune surveillance. Areas of particular strength include the development and function of B and T lymphocytes, antigen processing and presentation, dendritic cells, the immunobiology of endothelial cells, cytokines, innate immunity, B and T cell tolerance and memory, mechanisms of bacterial and viral pathogenesis, bacterial mobility, and viral targeting and replication. Many important human diseases are under study, with a particularly strong focus on the mechanisms of autoimmunity, allergy, and immunodeficiency (diabetes, systemic lupus erythematosus, multiple sclerosis, asthma, AIDS), and on important bacterial, viral, and fungal pathogens. Predoctoral training leading to the Ph.D. degree begins with formal course work and research rotations in the first year, followed by additional course work, teaching, and the qualifying exam in the second year. Thesis research begins the in summer of the first year and becomes the primary focus of activity after completion of the qualifying exam. Intensive training in the methods and logic of research are supplemented with a wide array of opportunities for scientific interaction, including seminars, retreats, and participation in national meetings. The average time required to fulfill all of the requirements for the Ph.D. degree is five years. The vast majority of graduates go on to do postdoctoral research, and many then obtain independent positions at academic or research institutions, or obtain research positions in biotechnology or pharmaceutical companies. Postdoctoral training is a three-year program that focuses intensively on research in the laboratory of one or more of the trainers. It is enriched by many opportunities for scientific interactions.